This invention relates to electroluminescent (EL) lamps and, in particular, to an EL panel made from PVDF/HFP resin. As used herein, an EL xe2x80x9cpanelxe2x80x9d is a single substrate including one or more luminous areas, wherein each luminous area is an EL xe2x80x9clampxe2x80x9d.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. Either the dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder between the dielectric layer and one electrode. The phosphor powder radiates light in the presence of a strong electric field, using very little current.
A modern (post-1990) EL lamp typically includes a transparent substrate of polyester (polyethylene terephthalate, PET) or polycarbonate having a thickness of about 7.0 mils (0.178 mm). A transparent, front electrode of indium tin oxide (ITO) is vacuum deposited onto the substrate to a thickness of 1000 xc3x85 or so. A phosphor layer is screen-printed over the front electrode and a dielectric layer is screen-printed over the phosphor layer. A rear electrode is screen-printed over the dielectric layer. A rear insulation layer may be added in the form of a screen-printed layer or a tape with an adhesive coating.
The inks used for screen-printing include a binder, a solvent, and a filler, wherein the filter determines the nature of the printed layer. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, or epoxy. A phosphor layer is typically screen-printed from a slurry (ink) containing a solvent, a binder, and doped zinc sulphide phosphor particles, such as described in U.S. Pat. No. 5,418,062 (Budd). A dielectric layer is typically screen-printed from a slurry (ink) containing a solvent, a binder, and barium titanate (BaTiO3) particles.
A rear (opaque) electrode is typically screen-printed from a slurry (ink) containing a solvent, a binder, and conductive particles such as silver, carbon or graphite, or mixtures thereof. When the solvent and binder for each layer are chemically the same or similar, there is chemical compatibility and good adhesion between adjoining layers. The respective layers are applied, e.g. by screen-printing or roll coating, and then cured or dried.
Thus summarized, the manufacture of EL lamps appears simplicity itself. Unfortunately, there are a few details that complicate the situation. Silver tends to migrate from the rear electrode toward the front electrode, causing black spots or shorts in a lamp. Thus, for higher performance EL lamps, subject to rugged environmental exposure at elevated temperature and humidity, silver is used for bus bars located away from the lamp areas rather than for the rear electrode.
A silver-based rear electrode has a lower resistivity than a carbon-based rear electrode. Thus, eliminating silver tends to limit the area of an EL panel because of non-uniformity in brightness across the face of a large area lamp with a carbon rear electrode. Placing a silver bus bar around the perimeter of a panel helps some but not nearly as much as placing a bus bar across the middle or the longest dimension of a panel. However, the silver from the bus bar will migrate through the rear electrode using the lamp materials of the prior art.
Most EL lamps are made in batches by screen-printing rather than being made continuously, e.g. by roll coating. Either way, a layer of material is typically formed as two or three successive layers due to the small amount of resin (binder) dissolved in the ink. It would significantly speed production, and reduce the amount of equipment necessary, if a layer could be formed in a single pass.
Lamps for different applications currently require different materials for the various layers. For example, the specifications for an automotive lamp are quite different from the specifications for a lamp in a wristwatch. The mechanical requirements for an automotive lamp are much more stringent than for a lamp in a wristwatch. For automotive applications, it is desirable that the lamp materials have a high softening temperature. Unfortunately, such materials generally have other characteristics that make them undesirable for EL lamps, e.g. low solubility. Low solubility means that the layer must be formed in several passes and the extra processing steps add to the cost of a panel.
An ITO-coated substrate is temperature sensitive due to shrinkage of the substrate at elevated temperatures. In many lamp panels, the substrate is xe2x80x9cpre-shrunkxe2x80x9d to stabilize the substrate for subsequent curing operations at high (150xc2x0 C.) temperature. A low film-forming temperature is therefore highly desirable for avoiding the need to pre-shrink the ITO coated substrate. Many materials with a low film-forming temperature are undesirable for EL lamps because of other characteristics of the materials.
Another problem is adhesion to the substrate in areas where there is ITO present and in other areas where the ITO has been removed. These problems can be overcome by the addition of adhesion promoting agents such as siloxane, e.g. Dow Corning Z6040. It is also known to add an acrylic resin to the ink to improve adhesion. Polymethyl methacrylate polymer (PMMA) and polyethyl methacrylate (PEMA) copolymer are compatible with PVDF-containing resins. The extra processing step of applying or including an adhesion promoter and the added material increase the cost of a panel.
A material that solves any one of the foregoing problems better than existing materials would be most welcome in the art. It has been discovered that a particular type of PVDF/HFP copolymer solves all the foregoing problems.
In view of the foregoing, it is therefore an object of the invention to provide a single construction for EL panels that addresses diverse markets, e.g. automotive, communication, and horology.
Another object of the invention is to provide an ink for making an EL panel wherein a complete layer is formed in a single pass.
A further object of the invention is to provide an EL lamp with a rear electrode containing silver for improved conductivity while exhibiting excellent environmental performance.
Another object of the invention is to provide an ink for making EL panels wherein the ink does not require pretreatment of a preceding layer or the addition of an adhesion promoter to an ink.
A further object of the invention is to provide an ink for EL panels wherein the ink does not require preshrinking of an ITO-coated substrate while retaining excellent high temperature environmental properties.
A further object of the invention is to provide an improved EL lamp in which at least one of the layers of the lamp includes a low molecular weight PVDF/HFP copolymer resin binder.
The foregoing objects are achieved in this invention in which EL panels are made with PVDF/HFP copolymer resin binder, in substantially an uncrosslinked form, with DMAC solvent and/or other higher boiling point solvents/latent solvents/extenders. The resin binder is characterized by a melt viscosity of 1.0-8.5 kPoise using an industry standard test (ASTM D3835). This viscosity is lower than the viscosity of PVDF/HFP copolymer resins used for other applications in the prior art.